The present invention generally relates to image-guided navigation. In particular, the present invention relates to a system and method for tracking the progress of insertion of a reducing rod in a bone.
Medical practitioners, such as doctors, surgeons, and other medical professionals, often rely upon technology when performing a medical procedure, such as image-guided surgery or examination. A tracking system may provide positioning information for the medical instrument with respect to the patient or a reference coordinate system, for example. A medical practitioner may refer to the tracking system to ascertain the position of the medical instrument when the instrument is not within the practitioner's line of sight. A tracking system may also aid in pre-surgical planning.
The tracking or navigation system allows the medical practitioner to visualize the patient's anatomy and track the position and orientation of the instrument. The medical practitioner may use the tracking system to determine when the instrument is positioned in a desired location.
Fractures of long bones such as femurs are typically repaired by inserting a rigid rod down the intramedullary canal of the bone. The rod is typically inserted into the end of one of the bone fragments. The rod is then passed across the fracture site and then inserted into the second bone fragment.
Generally, before the rigid rod is inserted into the two bone fragments, a reducing rod is first inserted into the bone and is the first device to cross the fracture site. As the reducing rod is inserted into the bone, the surgeon must simultaneously align the two bone fragments in order to ensure the reducing rod travels down the intramedullary canal of the second bone fragment. This alignment of the two bone fragments is known as reducing the fracture.
The surgeon may reduce the fracture with the aid of an image guided surgical techniques, such as frequent fluoroscopic x-ray images. When using image guided surgical techniques, it is desirable to virtually track the progress of the reducing rod and the positions of the two bone fragments.
The tracking of the reducing rod and bone fragments is typically performed by the utilization of sensors. A sensor is placed on the reducing rod and sensors are placed on at least the two major bone fragments. The tracking sensor may relate the distance between the tracking sensor and at least one of the two sensors placed on the bone fragments. In this way, the sensors may communicate distance information to a tracking device. The tracking device may then communicate the distance information to a computer system that includes a display such as a computer screen.
The computer screen displays a virtual image of the two bone fragments that may move relative to one another. In addition, the current position of the reducing rod tip relative to the bone fragments is displayed. The position of the reducing rod is determined by the tracking device and the computer system by placing the tracking sensor on the end of the reducing rod opposite the rod tip and utilizing a stored computer model of the reducing rod to calculate the position of the reducing rod tip.
However, several inaccuracies may cause significant errors to appear in the virtual image of the bone fragments and reducing rod. Several inaccuracies cause an inaccurate calculation of the position of the reducing rod tip. These inaccuracies include inaccuracies in calculating the pose (that is, position and orientation) of the tracking sensor attached to the reducing rod (opposite of the rod tip, as described above), inaccuracies in the attachment of the tracking sensor to the reducing rod, inaccuracies in the computer modeling of the reducing rod and flexion of the reducing rod while reducing the fragment.
These inaccuracies can cause severe virtual imaging errors. For example, when the reducing rod tip is superimposed on an image of the bone fragments on the computer display, as the reducing rod is passing through the bone shaft fragments, the reducing rod tip should always coincide with the central axis of the bone shaft fragments. However, because of the aforementioned inaccuracies, the computer often displays the existence of the reducing rod tip significantly outside of the bone shaft canal.
One manner in which similar inaccuracies encountered with surgical instruments have been addressed is in U.S. Pat. No. 6,478,802, entitled “Method and Apparatus for Display of an Image Guided Drill Bit,” issued to Kienzle et al., herein incorporated by reference in its entirety. The '802 patent discloses the utilization of a drill guide and a drill with localized emitters. A drill guide comprises localized emitters in known positions relative to the bore portion of the guide. The emitters assist in the calculation of the pose of the drill bit from the measured position data of both the drill guide and drill. In operation, graphic representations of the drill guide and bore are displayed. To generate the preferred drill bit representation, the location of the tip of the drill bit must be determined. The tip location is determined from the pose of the drill guide, the known location of a dimple on a drill guide and the position of the drill, in such a way that the representation of the drill bit tip is constrained to the bore of the drill guide.
However, the device and method disclosed in the '802 patent requires that the pose of the bore of the drill guide be known prior to use. This is a readily achieved with a precision-machined medical device with a tracking sensor attached in a known and fixed position relative to the bore portion. However, a tracking sensor may be attached to bone at any number of locations. In addition, the pose of the bone's central canal relative to the sensor is unknown. However, simply measuring the pose of an entire elongated medical device does not solve the inaccuracy problems discussed above. For example, the measured pose may be inaccurate, resulting in an appearance of the medical device outside of an area, or canal, to which the medical device should be constrained. In other words, measuring the pose of the entire medical device alone does not guarantee an accurate measurement and display of the position of the medical device.
In the display of the position of a reducing rod, as described above, the measurement of the pose of the reducing rod does not guarantee the accurate display of the position of the reducing rod, nor does the measurement of the pose of a tracking sensor attached to the bone. Therefore, a need currently exists for a system and method for more accurately tracking the progress of the insertion of a reducing rod into a bone.